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Published February 15, 2013 | public
Journal Article

Quantum chemical insights into the dissociation of nitric acid on the surface of aqueous electrolytes


Recent experiments in our laboratory have shown that the probability of gaseous HNO_3 deprotonation on the surface of water is dramatically enhanced by anions. Herein, we report a quantum chemical study of how a HNO_3 molecule transfers its proton upon approaching water clusters containing or not a chloride ion. We find that HNO_3 always binds to the outermost water molecules both via donating and accepting hydrogen-bonds, but the free energy barrier for subsequent proton transfer into the clusters is greatly reduced in the presence of Cl^−. As the dissociation of HNO_3 embedded in water clusters is barrierless, we infer that interfacial proton transfer to water is hindered by the cost of creating a cavity for NO_(3)^−. Our findings suggest that nearby anions catalyze HNO_3 dissociation by preorganizing interfacial water and drawing the proton—away from the incipient [H^(+)---NO_(3)^−] close ion-pairs generated at the interface. This catalytic mechanism would operate in the 1 mM Cl^− range (1 Cl− in ∼5.5 × 10^4 water molecules) covered by our experiments if weakly adsorbed HNO_3 were able to explore extended surface domains before desorbing or diffusing (undissociated) into bulk water.

Additional Information

© 2012 Wiley Periodicals, Inc. Issue published online: 22 January 2013; Article first published online: 31 May 2012; Manuscript Accepted: 9 April 2012; Manuscript Revised: 6 April 2012; Manuscript Received: 15 January 2012. This research was supported partially by a grant from the National Science Foundation to M.R.H. (NSF Grant AGS-964842).

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